HomeMy WebLinkAbout502153016 Geotech AssessmentStratum Group File: 5.15.19
1
PO Box 2546, Bellingham, Washington 98227
Phone: (360) 714-9409
October 30, 2021
Debbie Gussenhoven
261 Pascoe Avenue
Chehalis, WA 98532-8464
debbiegussenhoven@gmail.com
Re: Geologic Hazard Assessment
Jefferson County Parcel 502153016
Cormorant View Drive, Brinnon, WA
This geologic hazard evaluation was conducted to assess the risk of erosion and landslide
hazards on the subject property. It is our understanding that you are planning to construct a new
septic drainfield on the site and that a home will be later added on the site. The Jefferson County
Public Land Record shoreline slope stability map indicates that the western portion of the
property is within an area the map indicates as unstable.
Based on our geologic hazard assessment, the slopes on the property are not at risk of landslides
or rapid erosion. In particular the western portion of the site is not unstable. The moderate slopes
on the site are stable and will remain so, but map be subject to minor raveling and soil creep.
This geologic hazard assessment included a field inspection of the subject property and vicinity
including all slopes. Our evaluation also included review of the proposed septic plan, available
geologic mapping, lidar (light detecting and ranging) imagery and our own notes and
observations we have made in the vicinity of the site and at locations with similar geologic
conditions.
GENERAL GEOLOGY
The Geologic Map of Brinnon 7.5-minute Quadrangle, Jefferson and Kitsap Counties,
Washington (Polenz, 2012) indicates that the subject property is underlain by ice contact
deposits.
October 30, 2021
Jefferson County Parcel 502153016
Geologic Hazard Assessment
Stratum Group File: 10.1.21
2
Figure 1. Portion of geologic map (Polenz and others, 2012). Qgic = ice contact deposits, Qp = peat, Qga = advance
outwash, Qb = beach, Qpu = older quaternary sediments older than the Vashon glaciation, af = alluvial fan, Qam =
marsh, Qgt = glacial till, Evc = Crescent Formation.
The ice contact deposits were deposited during the late stages of the last glacial period as the
glacial ice tinned and melted away approximately 14,000 years ago. The subject property and
vicinity has a complex topography of depressions called kettles where blocks of ice within the
sediment melted creating multiple depressions (Figure 2).
Figure 2. Lidar bare earth imagery of property area. Note the kettle depressions throughout the area.
Site
October 30, 2021
Jefferson County Parcel 502153016
Geologic Hazard Assessment
Stratum Group File: 10.1.21
3
My observations are consistent with the above described mapping. Hand dug test pits and road
cut exposures on the site and in the vicinity consist of poorly sorted silt through cobbles and
boulders in a medium dense condition (Figure 3).
Figure 3. Poorly sorted sediment in a medium dense condition consistent with ice contact deposits on cut slope on
the south part of the site.
SITE SPECIFIC OBSERVATIONS
The property is accessed via Cormorant View Drive which crosses the western and southern
edge of the property. The proposed septic site is located on a nearly level area on the west side of
the site (Figure 4) between Cormorant View Drive and a moderate slope up to the upper central
portion of the site. The slope above the level area is a moderate slope that slopes up to the likely
building area on a level graded area above. The slope is forested with a mix stand of trees.
A graded gentle slope provides access from the southern edge of the property from Cormorant
View Drive up to a graded level area on the north central portion of the site. Moderate slopes
slope down to the west to and to the east from the level area. The slopes are forested with a mix
of trees.
Soils throughout the property consist of well drained silty to sandy gravel consistent with the site
being underlain by ice contact deposits. No seeps or springs are present and no indications of
incipient landslides were observed or suspected on the slopes.
October 30, 2021
Jefferson County Parcel 502153016
Geologic Hazard Assessment
Stratum Group File: 10.1.21
4
Figure 4. View of proposed septic area with slope to the upper building area.
GEOLOGIC HAZARD ASSESSMENT
The slopes on the property are underlain by well drained ice contact deposits. The slopes are a
result of in place ice wasting within the glacial sediment forming kettle topography with various
shaped depressions. I did not observe any evidence of previous, ongoing, or incipient slope
failures. I also did not observe any evidence of groundwater perching, seepage, or springs. There
are no geologic contacts underlying that site that could act as potential failure surfaces for
landslides. There is no surface water on the site and soils are well drained so there is no risk of
rapid erosion.
October 30, 2021
Jefferson County Parcel 502153016
Geologic Hazard Assessment
Stratum Group File: 10.1.21
5
CONCLUSIONS
Based on my geologic hazard assessment, the slopes on the property are not at risk of landslides
or rapid erosion. The proposed septic drainfield will not be at risk from landslides or erosion and
will not increase the risk of landslides on or off the property.
A home can be located on the level graded area on the central portion of the property. There is no
risk of landslides impacting the level graded proposed home site and development of the level
area with a home will not increase the risk of landslides on or off the property.
The home foundation footings should be located at least 10 feet back from the edge of any slopes
grater than 20 degrees in order to avoid potential differential settlement of the foundation.
Stormwater can readily be dispersed on site consistent with the Western Washington Stormwater
Manual.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding this assessment please contact our office at (360) 714-9409.
Sincerely yours,
Stratum Group
Dan McShane, L.E.G., M.Sc.
Licensed Engineering Geologist